WINTER FLOODING

ONE OF OUR DEADLIEST WINTER WEATHER HAZARDS IS FLOODING.
IN PENNSYLVANIA ONE USUALLY ASSOCIATES SNOW...ICE...AND
BITING COLD WITH WINTER. BUT SOMETIMES NATURE THROWS A CURVE
AT US WITH UNSEASONABLE WARMTH...AND WITH IT...RAIN.
A NUMBER OF DIFFERENT FACTORS WORK TOGETHER TO PRODUCE FLOODS
IN WINTER.
WHEN UNSEASONABLE WARMTH COMES TO THE REGION IT WILL OFTEN
MELT MUCH...IF NOT ALL OF THE SNOW ON THE GROUND IN THE
LOWER ELEVATIONS. THE MELTING SNOW WILL SATURATE THE GROUND
AND ALSO BEGIN TO SWELL THE RIVERS.
OFTEN THE WARMER AIR WILL RISE OVER COLDER AIR TRAPPED IN THE
REGION. AS THIS AIR RISES IT RESULTS IN CLOUDS AND RAIN...
WHICH WILL COMBINE WITH THE MELTING SNOW TO INCREASE FLOWS IN
THE RIVERS. WHEN THIS MELTING SNOW COMBINES WITH HEAVY RAINS
IT CAN PUT ENOUGH WATER INTO THE RIVERS TO SEND THEM OVER
THEIR BANKS.
SOME OF OUR WORST WINTER FLOODS ARE CAUSED BY INTENSE CYCLONES
THAT TRACK FROM THE OHIO VALLEY NORTHEAST UP THE SAINT LAWRENCE
VALLEY INTO CANADA. THESE STORMS BRING A LOT OF WARM AND MOIST
AIR INTO THE REGION FROM THE GULF OF MEXICO AND THE ATLANTIC...
BRINGING BOTH MILD TEMPERATURES AND HEAVY RAIN TO THE REGION.
IN JANUARY 1996...UNSEASONABLY WARM AIR AND HEAVY RAIN COMBINED
TO MELT A DEEP SNOWPACK IN PENNSYLVANIA. THE COMBINATION OF
MELTING SNOW AND HEAVY RAIN LED TO THE WORST FLOODING IN
PENNSYLVANIA SINCE HURRICANE AGNES IN 1972.
ANOTHER WINTER PROBLEM IS FLOODING CAUSED BY ICE JAMS
ON THE RIVERS. AS RIVER FLOWS INCREASE...WATER LEVELS RISE.
SINCE ICE THAT COVERS THE RIVERS IS LIGHTER THAN WATER IT WILL
TEND TO FLOAT. UNDER THE PRESSURE IT WILL OFTEN BREAK INTO HUGE
SLABS. THESE SLABS WILL THEN MOVE DOWNSTREAM IN THE CURRENT UNTIL
THEY RUN INTO AN OBSTRUCTION SUCH AS A BEND...ISLAND...OR
WIDE SHALLOW AREA. WHEN THIS HAPPENS THE ICE WILL OFTEN STOP
AND PILE UP INTO A JAM. WHEN THE FLOW OF THE RIVER IS BLOCKED BY
AN ICE JAM...THE WATER CAN OVERFLOW THE RIVER BANKS IN LESS
THAN AN HOUR AS IT TRIES TO GET AROUND THE ICE. AS THE WATER
RISES...THE PRESSURE CAN BREAK THE JAM AND RELEASE A SUDDEN SURGE
OF WATER AND ICE DOWN THE RIVER.
WHILE ICE JAMS OFTEN FORM IN THE SAME SPOTS YEAR AFTER YEAR IT
IS NEARLY IMPOSSIBLE TO PREDICT EXACTLY WHEN OR WHERE A JAM
WILL FORM...OR WHEN ONE WILL BREAK. SOMETIMES A JAM THAT FORMS
IN EARLY WINTER WILL STAY PUT MOST OF THE WINTER.
SEVERE FLOODING OF ROADS AND BUILT UP AREAS CAN ALSO OCCUR
WHEN MOUNDS OF PLOWED SNOW AND ICE BLOCK AND PLUG UP THE
GRATES AND STORM DRAINS...SO THE WATER FROM THE PAVEMENT
HAS NO WHERE TO GO. THIS STANDING WATER CAN CAUSE DANGEROUS
BLACK ICE IF IT FREEZES.